feat(client_libs): add Dart client lib (#4134)

Author: bednar

src/writeData/clients/Dart/description.md

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+For more detailed and up to date information check out the [GitHub Repository](https://github.com/influxdata/influxdb-client-dart).
+
+##### Use this package as a library
+
+For Dart run this command:
+
+```
+dart pub add influxdb_client
+```
+
+For Flutter run this command:
+
+```
+flutter pub add influxdb_client
+```
+
+This will add a line like this to your package's `pubspec.yaml`:
+
+```
+dependencies:
+  influxdb_client: ^2.2.0
+```

src/writeData/clients/Dart/dispose.example

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+client.close();

src/writeData/clients/Dart/execute.example

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+var query = '''
+  <%= query %>
+''';
+
+var queryService = client.getQueryService();
+
+var records = await queryService.query(query);
+await records.forEach((record) {
+  print('${record['_time']}: ${record['_field']} = ${record['_value']}');
+});

src/writeData/clients/Dart/executeFull.example

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+import 'dart:io' show Platform;
+
+import 'package:influxdb_client/api.dart';
+
+void main() async {
+
+  // You can generate an API token from the "API Tokens Tab" in the UI
+  var token = Platform.environment['INFLUX_TOKEN'];
+  var bucket = '<%= bucket %>';
+  var org = '<%= org %>';
+
+  var client = InfluxDBClient(
+      url: '<%= server %>',
+      token: token,
+      org: org,
+      bucket: bucket);
+
+  var query = '''
+    <%= query %>
+  ''';
+
+  var queryService = client.getQueryService();
+
+  var records = await queryService.query(query);
+  await records.forEach((record) {
+    print('${record['_time']}: ${record['_field']} = ${record['_value']}');
+  });
+
+  client.close();
+}

src/writeData/clients/Dart/index.ts

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+import logo from 'src/writeData/clients/Dart/logo.svg'
+import description from 'src/writeData/clients/Dart/description.md'
+import initialize from 'src/writeData/clients/Dart/initialize.example'
+import writeLP from 'src/writeData/clients/Dart/write.0.example'
+import writePoint from 'src/writeData/clients/Dart/write.1.example'
+import execute from 'src/writeData/clients/Dart/execute.example'
+import query from 'src/writeData/clients/Dart/query.example'
+import executeFull from 'src/writeData/clients/Dart/executeFull.example'
+import dispose from 'src/writeData/clients/Dart/dispose.example'
+
+export default register =>
+  register({
+    id: 'dart',
+    name: 'Dart',
+    description,
+    logo,
+    initialize,
+    write: [
+      {
+        title: 'Use InfluxDB Line Protocol to write data',
+        code: writeLP,
+      },
+      {
+        title: 'Use a Data Point to write data',
+        code: writePoint,
+      },
+    ],
+    execute,
+    query,
+    executeFull,
+    dispose,
+  })

src/writeData/clients/Dart/initialize.example

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+import 'dart:io' show Platform;
+
+import 'package:influxdb_client/api.dart';
+
+void main() async {
+
+  // You can generate an API token from the "API Tokens Tab" in the UI
+  var token = Platform.environment['INFLUX_TOKEN'];
+  var bucket = '<%= bucket %>';
+  var org = '<%= org %>';
+
+  var client = InfluxDBClient(
+      url: '<%= server %>',
+      token: token,
+      org: org,
+      bucket: bucket);
+}

src/writeData/clients/Dart/logo.svg

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+from(bucket: "<%= bucket %>") |> range(start: -1h)

src/writeData/clients/Dart/query.example

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+var record = 'mem,host=host1 used_percent=23.43234543';
+
+var writeService = client.getWriteService();
+await writeService.write(record).then((value) {
+  print('Write completed');
+}).catchError((exception) {
+  print(exception);
+});

src/writeData/clients/Dart/write.0.example

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+var point = Point('h2o')
+    .addTag('location', 'Prague')
+    .addField('level', 1.12345)
+    .time(DateTime.now().toUtc());
+
+var writeService = client.getWriteService();
+await writeService.write(point).then((value) {
+  print('Write completed');
+}).catchError((exception) {
+  print(exception);
+});